Firefighters across the U.S., many in full gear, climbed 110 flights of stairs Tuesday to pay tribute to the hundreds of emergency responders who were killed in the terrorist attacks of Sept. 11, 2001.

Some firefighters chose to start their march at precisely 8:46 a.m., the moment the first hijacked plane, American Airlines Flight 11, crashed into the North Tower of the World Trade Center in New York City, 17 years ago Tuesday. The 110 flights of stairs symbolize the building’s 110 floors.

Many of these climbs are held as annual events to keep the memory of the fallen heroes alive.

A photograph of firefighter Alexander Liczesky and recruit Timothy Solder of the Houston County Fire Department in Georgia went viral after it was shared by Chelsea Renee Farmer on Tuesday.

Liczesky, Solder and McKayla Conner, another recruit firefighter who wasn’t photographed, started their 110 flights of stairs at the Planet Fitness gym in Warner Robins, Georgia, at 8:46 a.m., a Houston County fire official confirmed with HuffPost.

Farmer was at the Georgia Planet Fitness, too, when the firefighters’ tribute began.

“I felt overwhelmed with pride for my country. Coming from a military family and having a family member who served and sacrificed his life fighting for our freedom, seeing this hit me hard,” Farmer told local CBS news affiliate 13WMAZ.

“It was a very special moment I will never forget, and I’m so glad I got to witness it. It makes me proud to be an American.”

A Planet Fitness employee in Marville, Tennessee, captured a similar moment in 2016 when Fairview city firefighter Jason Harding climbed a stair machine in full gear.

Gold’s Gym Linglestown in Harrisburg, Pennsylvania, reserved its stair climbing machines for people who wanted to complete a tribute to raise money for Tuesday’s Children, a nonprofit that supports the families of the victims of the 9/11 attacks, according to ABC 27 News.

Dylan Landuyt, a firefighter at the Progress Fire Company, showed up at the Harrisburg gym’s machines with his full gear on.

In San Antonio, emergency responders, including police officers and firefighters in full gear, climbed the Tower of the Americas building twice to honor the fallen heroes of 9/11, according to news station KSAT-TV.

The event is known as the San Antonio 110 9/11 Memorial Climb and has been held at least six times since 2001.

Firefighters, law enforcement officers and members of the public walk toward the Tower of the Americas for the sixth annual San Antonio 110 9/11 Memorial Climb. pic.twitter.com/9p5PNrxJq0

Firefighters in Colorado marched up the Manitou Incline hiking trail, which reaches an elevation of 2,000 feet, on Tuesday, according to KOAA-TV.

Meanwhile, more than 2,300 people from 26 states climbed the staircase at the Red Rock Amphitheatre near Denver nine times to replicate the number of steps in the World Trade Center tower, Fox31 Denver reported.